We will study roles of estrogen in the pathophysiology of intracranial aneurysms. Incidence of intracranial aneurysms and subarachnoid hemorrhage are higher in women than in men. However, the female preponderance in incidence of intracranial aneurysms and subarachnoid hemorrhage becomes significant only after the menopausal age. Furthermore, estrogen replacement therapy reduces the risk for aneurysmal subarachnoid hemorrhage in post-menopausal women. Our preliminary data using a newly developed mouse model of intracranial aneurysm show that ovariectomy (surgical menopause) increased the incidence of intracranial aneurysms in female mice. And, estrogen treatment reduced the incidence of aneurysms in ovariectomized mice. The protective effects of exogenous estrogen were significantly reduced when the estrogen therapy did not start immediately after ovariectomy (delayed estrogen replacement), consistent with the timing hypothesis, which states that responses of vascular and inflammatory cells to estrogen are altered after a long period of hypoestrogenicity. Our general hypothesis is that estrogen has protective effects against the formation of intracranial aneurysms. Furthermore, we hypothesize that the protective effects of estrogen are through hematopoietic cells (primarily inflammatory cells). Aim 1 is to determine contributions of estrogen receptor-a(ERa) and estrogen receptor- (ER) to the formation of intracranial aneurysms. We hypothesize that the protective effects of estrogen are primarily mediated by estrogen receptor-a (ERa). We will utilize ERa knockout mice, ER knockout mice, and receptor subtype specific agonists. Aim 2 is to determine the cell type that is mediating estrogen's protective effects against the formation of intracranial aneurysms. We hypothesize that the protective effects of estrogen against aneurysm formation are primarily through hematopoietic cells (primarily inflammatory cells). Aim 3 is to identify the cell type and receptor subtype responsible for the reduced protective effects of estrogen after a prolonged period of hypoestrogenicity. We hypothesize that the loss of protective effects of estrogen after a prolonged period of hypoestrogenicity is due to the lack of estrogen stimulation on ERa in hematopoietic cells during the prolonged hypoestrogenic period. Results will provide new insights into the roles of estrogen in the pathophysiology of intracranial aneurysms and mechanisms for the gender difference. As a first step, we will focus on roles of estrogen in the formation of intracranial aneurysms in post-menopausal women. This study will be a basis for future studies to develop new therapies that target specific estrogen receptor subtype in specific tissues for the prevention of growth and rupture of intracranial aneurysms, especially in post-menopausal women who are at high risk for aneurysmal subarachnoid hemorrhage.